Oxygen scavenging method



United States Patent 3,387,969 OXYGEN SCAVENGING METHOD Stanley B.Skladzien, Elmhurst, Ill., assignor to the United States of America asrepresented by the Chairman of the United States Atomic EnergyCommission Filed Aug. 2, 1965, Ser. No. 476,767 3 Claims. (Cl. 75-66)The invention relates to a method for removing oxide from molten alkalimetal and maintaining the alkali metal in an oxide free condition.

In many alkali metal systems it is very important to keep oxide contentas low as possible. The presence of oxide in the alkali metalsubstantially increases the corrosiveness of the alkali metal to manymetals. Such metals are necessarily used for containers and othermechanical parts.

There are a number of presently available devices that can accomplishthis purpose. All that is needed is a container filled with materialthat can scavenge oxide from the molten alkali metal. This scavenger canbe in any bulky form offering a large amount of surface to the moltenalkali metal.

However, these devices suffer from a number of severe shortcomings. Itis extremely difiicult to determine when the scavenger material isexhausted and severe damage may result before this becomes known.Further, these devices are normally fabricated into a closed system sothat the system must be opened to replace a unit. With the knowntendency of alkali metal to wet and adhere to metal surfaces, and itstendency to catch fire in air, such a change is a major undertaking.

It is an object of this invention to provide a method by which theoxygen scavenging material is continuously and automaticallyregenerated.

It is also an object of this invention to provide a deoxidizing devicewhich will function uniformly during its entire life.

It is a further object of this invention to provide a device whicheliminates the need of frequent, troublesome replacements.

This is accomplished by providing as the scavenger, a continuous thinzirconium-containing membrane, one surface of which is in contact withthe alkali metal, the other with a molten calcium containing metal,which regenerates the membrane by virtue of the unique diffusion ofoxide therethrough.

In the drawing:

FIGURE 1 is a vertical, partially sectional view of the apparatus; and

FIGURE 2 is an enlarged vertical sectional view of the bottom portion ofFIGURE 1.

Referring to FIGURE 1, the containing vessel 2 consists of an uppershell 4 and a lower shell 6, each shell being constructed of A" thick(AISI designation) 304 stainless steel. Shells 4 and 6 are cylinders 16"OD, 15" high. At their mating ends they are joined to flange-s 8, 19"OD. Upper shell 4 is completed with conical top 10, rising 2" and joinedto nipple 12, 3 OD, 2" high terminating with 7 /2 diameter top flange14.

Lower shell 6 is closed by skewed conical bottom 16, through which a 3"long, 2" OD nipple 18 is axially positioned. Nipple 18 is connected toflange 20, which is 7 /2" OD. At the lowest point of conical bottom 16,drain pipe 22, a length of OD tubing extends downward. At some point onpipe 22, a valve (not shown) permits the contents of containing vessel 2to be drained, when desired. Inlet line 24 and outlet line 26 areattached to the lower shell 6 and upper shell 4 respectively, atopposite sides, 11" from the mating surfaces of flanges 8.

37 tubes 32 are provided, each of which is a membrane 3,387,;369Patented June 11, 1968 as mentioned above and is formed of zirconium ora zirconium alloy such as Zircaloy-II. Each tube is 12" long and thickand has an outside diameter of 1". The tubes are parallel and have theiraxis on the corners of equilateral triangles 1 on a side, so that eachtube, except the outermost ones, is surrounded by six tubes that have anaxis spacing of 1 from one another and from said each tube. The tubes 37as a group present the general outline of a regular hexagon. The tubesare inserted into tube sheets 34, which take the form of discs of thick,14" diameter and are formed of zirconium 0r zirconium alloy. Closure iscompleted by inert arc welding. On each tube sheet 34, circular bands 361" wide, 16" thick extend outward. Upper manifold 38 is a A shallowconical vessel, to which is attached upper conduit 40, which is A inthickness and 2 /2" in OD and is formed of zirconium or zirconium alloy.The upper end of conduit 40 leads to a molten calcium alloy storagevessel (not shown). Flow is controlled by a conventional valve (notshown). Lower manifold 42 is a conical vessel of zirconium or zirconiumalloy, deeply sloped to avoid collection of sediment, terminating inoutlet conduit 44 thick, 1 /2" OD. This conduit 44 terminates in avalve, symbolically shown, which regulates the flow of molten metal to areceiving vessel (not shown).

After assembly, discs 52 of zirconium or zirconium alloy, /8 thick, 6/2" OD, are positioned on upper conduit 40 and outlet conduit 44 andwelded in place by inert arc welding. Flanges 14 and 20 are grooved toaccommodate hollow zirconium 0 rings 50. Against these 0 rings 50 arepositioned discs 52 welded in position in situ, using inert arc welding.Closure is completed by positioning upper centering flange 54 and lowercentering flange 56 against discs 52. Bolts 58 complete the seal.Heating is accomplished by conventional means, preferably by inductiveheat.

Obviously, other simple modifications could be used to carry out theteachings of the patent. All that is necessary is to provideapproximately 20 30 square feet of thin walled zirconium or zirconiumalloy such as Zircaloy- II, with contact on one surface with calciummetal and the alkali metal to be cleaned on the other surface.

Due to the ease of brittle compound formation between zirconium and ironalloys, such as stainless, all joints between these metals must bemechanical. Welding such members is not desirable.

Optimum operating temperature is l200l300 F. This temperature is belowthe melting point of calcium, so that it is desirable to add sufficientmagnesium to approach one of the eutectics, in the neighborhood of 21%magnesium by weight. A range of 1530% magnesium should operatesatisfactorily.

The present invention is employed to remove oxygen from sodium that isrelatively low in oxygen contamination, for example, about 10 parts permillion. This condition will have been reached by one or two treatmentsof the sodium in a cold trap that has wire mesh on which the oxygen istrapped out when the sodium temperatures drop below the saturationtemperature, but not below about 225 F.

The need to employ the present invention to obtain sodium with less than10 ppm. oxygen comes from the trend to higher temperatures and use ofrefractory metals for jackets in sodium reactor systems.

It is to be understood that the invention is not to be limited to thespecific design shown or the details presented, but only by the scope ofthe appended claims.

The embodiment of the invention in which an exclusive property orprivilege is claimed are define-d as follows:

1. A method of removing oxide from alkali metal comprising flowing saidalkali metal past a membrane of 3 y v4-1 zirconium or zirconium alloymaintained at an elevated References Cited 2. The method of claim 1where the temperature is 2813O19 11/1957 Olson 75 66 X maintained at1200 130O F 5 2,815,277 12/1957 Bruggeman et a1. 75-66 3. The method ofclaim 1 where the calcium containing alloy consists essentially ofcalcium and 15-30 weight HYLAND BIZOT Pnmary f percent of magnesium. H.W. TARRING, Assistant Examiner.

1. A METHOD OF REMOVING OXIDE FROM ALKALI METAL COMPRISING FLOWING SAIDALKALI METAL PAST A MEMBRANE OF ZIRCONIUM OR ZIRCONIUM ALLOY MAINTAINEDAT AN ELEVATED TEMPERATURE, MEANWHILE FLOWING A CALCIUM-CONTAINING ALLOYON THE OTHER SIDE OF SAID MEMBRANE.